 Organometallic chemistry is extensively used in catalysis for the synthesis of small molecules for large molecules like polymers and in day to day applications in the laboratory. However, one always wonders whether it can be used for other purposes and whether it has been used and this question is answered in today's lecture and presentation here. We will talk about some special properties and applications. In fact, the number of instances where organometallics have been used are quite large, but we will consider only a few of the topics like sensors, metal organic chemical vapour deposition and non-linear optics. So, let us start off with the special properties of ferrocene which make it really suited for electrochemical applications. If you remember the molecular orbital of ferrocene is such that the highest occupied molecular orbital is completely delocalized on the iron atom of the ferrocene. So, it is at the iron atom that the electron density is maximum in the ferrocene molecule and if you remove an electron it is the iron which is depleted of the electron primarily and it is in a non-bonding orbital, an orbital which is not interacting significantly with the cyclopentadienyl rings. In spite of that, we will see that there are some effects of the cyclopentadienyl ring, but this non-bonding nature of the highest occupied molecule orbital gives the iron atom tendency to have very little change when it goes from ferrocene to the ferrocenium cation. So, the ferrocenium cation which is pictured here has got very similar iron carbon distances, it has got very similar carbon carbon bond distances. So, it allows for reversible electron transfers and this property of reversibly transferring an electron to a molecule is possible only if the highest occupied molecule orbital is non-bonding and that is a completion ferrocene. You might always wonder so if it is not having changes in the bond distances and if it is going to be a reversible transfer, what is the use? If you recollect in electrochemistry, we usually use the standard Kalamel electrode as a reference point and in some systems where the standard Kalamel electrode cannot be used, it is necessary to use a secondary standard and ferrocene serves exactly that purpose. In many instances, ferrocene can be even used as an internal standard. That means during the course of electrochemistry, one can in fact add ferrocene into the solution, carry out the electrochemical experiment without ferrocene interfering with the processes that are going on. In other instances, ferrocene actually functions as an electrochemical catalyst. In other words, it permits the rapid transfer of an electron from say the electrode to the solution to the species in solution. So, this kind of a rapid electron transfer or the catalysis of the electron transfer is very important for electrochemical processes. On my right side, I have shown you couple of electrode potentials and all of them belong to the ferrocenium cation being reduced to the ferrocene neutral molecule in a variety of different solvents. You will notice that although there is a variation in this potential, as long as the solvent is kept reasonably constant. In other words, if it is not contaminated with water or very polar compound, this voltage is reasonably close to the value that is indicated here. So, one can be sure that ferrocene can be used as a secondary standard in electrochemical applications. I told you that there are some effects of the substituent on the cyclopentadienyl ring. Here, I have shown you four different molecules with respect to ferrocene, nem cation ferrocene. This electrode potential is marked as 0 volts because it is reference to this ferrocenium ferrocene couple. You will notice that if I add a very electron donating substituent such as a methyl group on the cyclopentadienyl ring, in this particular case, of course, we have added 10 methyl groups, 5 on the cyclopentadienyl ring on the top and 5 on the cyclopentadienyl ring on the bottom. Because we have added so many electron donating groups, this molecule becomes very difficult to reduce. So, it requires about minus 0.55 volts with respect to ferrocene to get reduced from the cationic species to the neutral species. So, here we are talking about a single electron transfer. Now, if you do a substitution on the ring with an electron withdrawing group as in the case that I have shown here, that is an acetyl group is added. Then of course, it becomes easier to add the electron. Now, this seems to be slightly contradicting what I started out with namely, the electron is completely localized from the ion and it is in a non-bonding orbital. And this would require that you have no changes in the electrode potential when you add or remove an electron withdrawing group. But nevertheless, you can see that there are changes, but all these changes happen to be in a reversible fashion. In other words, the acetyl ferrocene always has potential of 0.27 volts with respect to ferrocenium ferrocene cation coupled. So, if you make it the diacetyl group as I have indicated here, then of course, it becomes even more easy to pump an electron and it gets reduced at 0.49 volts with respect to ferrocenium ferrocene cation coupled. So, you can see that there are substituting effects, but they all are still reversible electron transfers. And one can depend on these chemical shift or electrode electrochemical potentials changing in a systematic fashion. So, this has been utilized for functionalizing an electrode. As I have shown you here, I have an electrode which is some many times a glassy carbon electrode which is attached to a cyclopentadienyl containing molecule. We talked all the while about the ferrocene, but it can be several cyclopentadienyl complexes. They can be half sandwich complexes as I have indicated here or this cyclopentadienyl sandwich complexes as I have shown on the left hand side. So, if I have acetylide moiety attached to the cyclopentadienyl ring, then depending on the group that is there on the terminal position of the cyclopentadienyl group, it can be oxidized and oxidatively coupled to the glassy carbon electrode. This has been very recently shown to be a easy method for functionalizing the glassy carbon electrode. So, this is a glassy carbon electrode which is generally an inert electrode where you can carry out electron transfers, but the transfer itself has a high barrier. But when you attach an organometallic moiety as I have shown here, then the electron transfer between the reduced and the oxidized species. The reduced and the oxidized species transfer electrons to the organometallic species and so transfer the electron to the glassy carbon electrode. So, this transfer turns out to be a very facile transfer and the electrochemical reaction is facilitated because of the functionalized electrode. Now, this is exactly the principle on which the glucose sensor for example, has been designed. These are amperometric sensors. In other words, they measure the current flow that is there in the electrochemical setup in order to detect the presence of a species and to quantify the species. So, you have an enzyme which specifically oxidizes glucose, but the enzyme itself is not capable of transferring that electron to the electrode. So, you need a modified electrode and the electrode is modified using ferrocene substituted species. This is what is allowing Claremont and co-workers to even develop an implantable version of a glucose sensor which would be extremely useful of course for patients suffering from diabetes. They would have real time monitoring of the glucose in the blood. So, this is possible because you can have an electrode which is functionalized with say ferrocene or a substituent of a ferrocene and the glucose is oxidized, glucose oxidized enzyme oxidizes the glucose to a gluconolactone. And this gluconolactone when it is formed it releases two electrons and these two electrons are now transferred to the enzyme. The enzyme in turn transfers it to the electrode, but a direct transfer turns out to be difficult and it is the ferrocene which is attached to the glassy carbon electrode or a suitable electrode is a one which is necessary for conveying this electron from the enzyme to the electrode. So, you have a very easy way of measuring the amount of glucose that is present in the system using this type of an electrochemical setup. Normally one usually one measures the current that is being generated as a result of this electrochemical reaction and this is conveniently monitored. Now, you can you might have realized that if I can use ferrocene as a catalyst for an electrochemical reaction it must be a way of coating this electrode with the organometallic species. And one can use a single molecule functionalized and coated on to the electrode or one can think of polymers which are either polymers because they connected through the organic ligand that is a cyclopentadienyl ligand that is present on the ion or one can think of connecting it through the metal in other words is it possible for us to generate a polymeric species where the metal atoms are linked one after the other. So, here I have shown for you some organometallic polymers and these organometallic polymers are generated in a couple of different ways. In the first in this first example that I have shown to you there is a unique way of linking the polymers and that is what is called a ring opening polymerization it is analogous to the ring opening polymerization. But in this case it is it is assisted by the strain that is caused when you link two cyclopentadienyl rings through a very small bridge. So, this bridge opens up and when this bridge opens up it can link to another molecule in such a way that you can release the strain. And so the strain that was there because of this acute angle which should have been 180 degrees and it is made acute because of the bridge when a ring opens you end up with a very nice polymeric species where there is no strain. And ferrocene is linked to another molecule to another molecule of the ferrocene in a nice chain. So, this is the chain that is generated and this chain propagates. Now, what is the advantage of such a system? You have a processable polymer and this polymer can be now coated on to the electron. And so it forms an electro catalyst which can reversibly transfer electrons. And the transfer between these two systems also is reasonably facile. So, that one can have a very nice electro chemical reactions. Now, this transfer of an electron from one ion atom to the other although I have presented it as if it is very easy. It is not always a completely reversible electron transfer. And the system where it is completely reversible is the bisphylvalene diion. This is shown here. This is called the bisphylvalene diion molecule where the two cyclopentadienyl units are directly conjugated. And if you if both ion atoms are linked through both cyclopentadienyl rings then it forms a completely delocalized system. So, much so the oxidation state of each ion atom is 2.5. And no property of the ion atom distinguishes it as a plus 3 and plus 2 unit. They are completely delocalized and completely equivalent in any time scale that you can think of. So, it is not as if an electron is hopping back and forth. It is the electron is sitting in a delocalized molecular orbital on the two ion atoms. Now, the molecular weights of these molecules when they are linked directly to each other turns out to be reasonably small. But molecules like this species can be fairly large. They can be as high as 10 power 5 to 10 power 7 voltants. And still the electrochemical properties of these molecules are maintained and they are weak semi conducting polymers. So, in other words these organometallic polymers are useful species. They turn out to be extremely valuable for electro chemically coating some electrodes. And also in some specialty applications where you need a conducting surface for a particular application. Here is another way a second way which I wanted to illustrate for making an organometallic polymer. In this instance you have a molecule which is a sandwich structure. But the sandwich structure has two different types of breads on either side of the metal atom which is in the center. Because you have a aromatic ring the aromatic ring has got two chlorine atoms. And you will remember that because of electron withdrawal from the aromatic ring it is now possible to carry out nucleophilic substitution of the aromatic ring with the phenoxy group. So, any nucleophile will do. But if you have a phenol which is a diol then a bis phenolic compound can do substitution with two different molecules of the ruthenosine which is now a polymeric species. Because you are going to repeat this unit several times. And so you have a nice processable polymer which is linked directly which is linking directly two different ruthenosine atoms or entities. So, this kind of substitution reaction turns out to be extremely useful. And it can be used for several applications in electrochemistry. When it comes to very small small say when it comes to an application which requires a very small contact people have been thinking about making molecular wires. In other words if I attach two electrodes through only a single molecular entity then I can have a molecular wire. But very often these single molecules do not conduct electricity and they turn out to be very poor conductors. One of the early studies which successfully made a molecule which almost look like a molecular wire starts out with a platinum two species which is a cyanocomplex. Although we do not consider this entity as strictly as an organometallic compound it illustrates the purpose for it illustrates the principle that we are talking about and that is the making of a molecular conductor. So, this platinum two plus unit can be partially oxidized with bromine to generate this oxidized species which turns out to be extremely interesting. Here is the oxidized species and here is the neutral species. The neutral species the platinum the p t c n 4 2 minus units are separated by 3.3 angstroms. The distance is significantly reduced when you oxidize the platinum. Oxidization of the platinum leads to shorter platinum-platinum interactions. This distance turns out to be reduced significantly and that has been attributed to removal of electron density from a d z square orbital which is along this axis. So, removal of an electron from this axis and formation of a platinum-platinum bond along this axis leads to the formation of a molecular conductor because it is only going to be a half filled orbital. Both electrons are not removed this is a partial oxidation and this gives you exactly the features of a conducting material. Turns out that this can be made with a variety of different oxidants with platinum oxidized in such a way that the anionic species is not there, but the cation is present in a non stoichiometric amount. Whatever be the procedure you end up with a copper colored a metal colored compound which has got a reasonably high conductivity. Of course, this does not have the good properties of the polymer that we talked about earlier, but nevertheless this is a molecular wire because the conduction is extremely high along this axis and this conductivity is also anisotropic. Very clear indication of the fact that it is delocalization of the electrons on the platinum along one particular axis. Now, in a very dramatic development recently, palladium molecular wires have been made. Now, turns out that platinum turns out that platinum was the only example where such an molecular wire was made and here for the first time palladium has been utilized to make a molecular wire. Once again palladium 2 plus complex is a complex you start out with and these molecules are pictured on the top. These are discrete molecules. They are stacked, but nevertheless they are not aligned with each other in the solid state. So, they are palladium 2 plus complexes, but if you oxidize them a very strange thing happens. A palladium palladium bond happen is formed just like you have in the case of the platinum. Now, there are two different things that are happening. One is that the aromatic ligands which are interacting with the palladium align themselves and also the palladium atoms are aligned so much so that this compound is likely to be a semiconductor or a conductor at best. Let us take a look at the oxidation reactions that have been carried out. This was an interesting case of a dinuclear system undergoing oxidative addition when the researchers Tobias Ritter used PHICL 2. PHICL 2 has a tendency to become PHI, PHI and the CL 2 oxidizes the palladium. During the course of this oxidation each palladium atom which was originally a palladium 2 plus center became a palladium 3 plus center, but because there are two electrons coming from the two chlorines you have both palladiums oxidize to palladium 3 plus center. You will notice that there is a palladium carbon bond in this molecule. So, that accounts for the second source of electrons for this molecule. So, you have palladium 2 plus to begin with and then you have three anionic ligands. So, you have a palladium 3 plus dinuclear oxidized center and surprisingly this molecule when you heat it, it only gives you a reductive elimination where this chlorine is removed along with this palladium along with breakage of this palladium carbon bond and formation of a chlorine carbon bond which gives you this molecule two units of this molecule. So, this is the case of a simple oxidative addition and a reductive elimination. Surprisingly when the when the workers used researchers used xenon fluoride, they formed this very interesting molecule which had counter ions of fluoride which is what you would expect, but a very strange alignment of the palladiums and the formation of a palladium palladium bond. This palladium palladium bond is close to 600 atoms long in the crystals that are available. They are so long that they are long needle like crystals which are metallic above 200 k and it is very clear that there is delocalization of the palladium deorbitals in such a way that you have this nice molecular conductor being generated because of this orientation. So, this is the same molecule that we showed you earlier and here it is shown as a line diagram. Now, let us move on from electrical properties to electronic properties of organometallics. The optical properties of organometallics are very similar to the optical properties of many organic compounds. More often than not materials respond to light in a linear fashion. If you have a light falling on a molecule, the molecule scatters the light and that light comes out with a lower intensity. It also transmits the light and that also comes out with a lower intensity because some of the light, some of the photons are absorbed by the molecule if they have the right energy to excite an electron from the ground state to the excited state. In some instances, in a few instances when you can have excitation, the excited electron coming back to the ground state, you can have fluorescence or phosphorescence. Because there are some changes in the vibrational states of the excited state molecule, you do have changes in the wavelength of the light. Usually, the light that comes out is of a longer wavelength. So, in most properties that you observe for molecules, the incident light's frequency and the light that is transmitted or scattered is usually of the same wavelength or of a longer wavelength. There are very few instances where molecules interact with the light in a non-linear fashion. And here also the intensity of the light that is coming out is of a lower intensity, but it turns out to be half the wavelength of the light that is incident on the molecule. Now, as the energy is inversely proportional to the wavelength, you will notice that the light that is coming out is of a higher energy than the light that is coming in. And so, the intensity obviously this can happen if you have two photons coming in and both photons have interacted in such a way in the molecule such that lower wavelength or more energy containing photon is coming out from the molecule. During these processes, all the other processes that we talked about, fluorescence, transmittance of light, etcetera, all of them scattering of light, all of them are happening. But in addition to these properties, you have this non-linear response. Now, this non-linear response is very interesting because you can use light of a low wavelength to convert it to a light of even smaller wavelength, but higher energy. So, this turns out to be very interesting and one has to maximize it and the whole race or the challenge is to maximize it and the intensity of this half wavelength light that is coming out. So, how does one do it? People have been working on this particular property for a long time now and a few simple thumb rules can be derived from what is called a two state model. According to the two state model, it is the difference in the dipole moment between the ground state and the excited state that is important. So, can we manipulate organometallic molecules such that they have a very large change in the dipole moment? This can be done with organic molecules also and it has been done and what one can do with organometallic molecules however is slightly different and that is what we are going to talk about. A second point that one has to note is that in the solid state, it should not be a centrosymmetric solid which will result in cancellation of these dipole moments and result in a zero response for the non-linear optical property. So, this non-linear optical property is measured as the coefficient beta and if the incident light is having a wavelength of 1064 nanometers it is indicated as a superscript because the non-linear response is dependent on the wavelength of the light that is coming in. So, if you have a 1064 nanometer light the beta value would be different from what you have if you have a 532 nanometer light that is coming in. Usually these measurements are made with high powered lasers because the intensity of the low wavelength high energy light that is coming out is reasonably small. So, you need a laser in order to measure the wavelength of intensity and wavelength of the light that is coming out. So, here are two molecules and the first principle that one can see is that when you increase the conjugation and the conjugation has been increased by adding this typhoon group. So, you have one typhoon group here, here you have two typhoon groups and you will notice you have two different groups. We will make it a donor and an acceptor. So, this donor acceptor units are separated by a larger distance in this molecule and that leads to a larger value for the non-linear response. So, you can see that this is just one example, but a multitude of examples can be given where it is very clear that if one increases the conjugation between the donor and the acceptor then it is possible to increase the non-linear response in the molecule. The second principle that is clear is that if one has a better acceptor or a better donor it is possible to have a better non-linear response. I am again illustrating it with a similar molecule, but you will notice that there is a tremendous increase in the non-linear optical property by attaching a molecule next to the Sino group which makes the Sino group a better acceptor. So, this is an acceptor to start with, but this acceptor is much better. So, because of the increase in the accepting property of the ferrocene half sandwich that is attached here, you have been able to increase the non-linear response by more than 2 fold close to 3 fold increase in the non-linear response has been achieved. So, these are two simple principles which I have shown to you using organometallic examples, but it is clear that some generalizations can be made on the basis of a large number of studies that have been done. Firstly, conjugation can be increased in order to achieve larger non-linear response and this is increased very readily in organometallic molecules. Secondly, they have to be stable because high intensity lasers come in and so if the molecules are not very stable on heating, they can decompose and you lose the response. So, far the best molecule or the unit that has been found to be a donor has been this bis diphenyl phosphinoethane moiety attached to a ruthenium center and the donors are better as you go down the group. It is also possible that donor acceptor properties can be changed by oxidation that is one advantage in an organometallic non-linear optical material because you can make molecular switches by reducing and oxidizing the ruthenium center. You would be changing the redox properties of the molecule in question. Now, in general a 3D transition metal is better than a 4D or a 5D transition metal and because you can polarize the 4D and 5D transition metals, one would have expected the opposite to be true, but in general it has been found the 3D transition metals are better. So, the organometallic compounds provide a very clear advantage for making new non-linear optical materials and this has been reviewed several times in the recent literature. Now, we proceed to properties of materials other properties of materials and let us take a look at how some materials are made using chemical vapor deposition. This is a root which is used to make for example, titanium nitride which is a gold colored compound it can be used for coating watches or ornaments and they may look like gold, but they are not really gold and this material titanium nitride is extremely hard it is conducting it looks very much like a metal. But unfortunately if you want to coat a metal or a material with titanium nitride it is not easy to carry out that process because it is not a volatile compound, but you can take an organometallic compound which is volatile. So, you take a volatile organometallic compound and then you decompose it on a substrate leaving behind the desired material which is a metal or a metal nitride a carbide or an oxide. All these possibilities are have been realized and the trick is to use an organometallic compound where the metal x bond whether it is to nitrogen or to carbon or to oxygen the metal x bond has to be weak. You can generate the desired species on the substrate and if you have reasonably easy ways of decomposing the material then the kinetic parameters are suitable for carrying out metal organic chemical vapor deposition and you do not have a very high temperature that is required for carrying out this chemical vapor deposition. The advantage of chemical vapor deposition is that you need very little post processing of the material and if you can make a molecule which has got both metal and nitrogen metal and carbon then you can have what is called a single source precursor for carrying out the cycle of chemical vapor deposition. This turns out to be an extremely useful parameter because if you have two different process materials giving you the metal and giving you the nitrogen then the processes for the required for controlling the two molecules at the substrate become very difficult So, let us take a look at metals if you want to deposit a metal for example you could take a metal allylic complex. These metal allylic complexes have got a very easy decomposition route they undergo reductive elimination of the two allyl groups and form this 1, 5 hexadiene. So, these are 1, 5 hexadienes so you can see that they are 1, 5 hexadienes and they are easily formed on decomposition of a divalent metal allylic complex. These species even if there are other l n molecules they turn out to be reasonably volatile. So, at very low temperatures you can volatilize this precursor and in a substrate which is kept hot it undergoes decomposition and deposits the metal in a pure form because this molecule is volatile it will be lost very readily and you end up with the deposited material which is in a pure state. One can not only use metal allyls one can even use metal alkyl species and in this case the decomposition might be slightly more complex. You can have a variety of decomposition routes you can have the ethyl group here. This ethyl group reacting with one of the hydrogens here and giving you ethane and an olefin complex and in a subsequent step the olefin might be lost and the metal might be deposited on the substrate. In another scenario one can also think of decomposing the molecule in such a way that the 2 ethyl groups are attached to each other and bond is formed and butane is eliminated. So, either way it is easy to make metals on top of a substrate using chemical vapor deposition of metal alkyl compounds. Here is a schematic representation of how one would do this reaction or how one would carry out. You can have this metal alkyl species sitting in a container which is volatilized using some heat and sometimes it is also nebulized using an inert gas and this carries this material on to a glass tube where the substrate is kept. A simple substrate would be a quartz substrate and usually this kept hot the temperature of this furnace is kept at a temperature above the temperature at which this material would decompose. So, it decomposes only here the organometallic species decomposes at this point deposits the metal on top of the substrate and the organic compounds are pumped out and the liquid nitrogen traps collect the volatiles and the organic compounds that come out. The whole thing is kept under high vacuum usually and in such a way that the organic species do not stay in the region where you want to have pure metal deposited on the substrate. Here is an example where you have some semiconductors made these semiconductors are called 3 5 semiconductors or 2 4 semiconductors and many of them have been made or deposited on a substrate using this chemical vapor deposition method. The temperatures are rather high it is about 600 degrees, but relatively lower than what you would end up with if you had to use a completely inorganic route for making these molecules or making these alloys. So, here is a gallium arsenide deposition using trimethyl gallium which is an organometallic species. You can also have a single source precursor which I was telling you about where there is a gallium arsenide bond and you can vaporize this gallium arsenide bond directly on to the substrate and deposit the required amount of gallium arsenide. The advantage of using a single source precursor over a 2 source precursor is that the vapor pressures of these 2 molecules are different and as a result it would be very difficult to adjust the amounts of gallium and arsenide and you should not end up with excess of arsenic or excess of gallium on the substrate. So, to avoid that a single source precursor is always preferred. One can also dope for example, here in the case of indium phosphide one has doped it with a terbium and this terbium is delivered as a cyclopentadienyl complex. So, once again you see that an organometallic lanthanide, organo lanthanide is used in order to dope the semiconductor appropriately. So, it terbium terbium have all been used as dopants and as organometallic precursors and here is a 2, 4 semiconductor which is also generated using an organometallic compound. So, you can see that chemical vapor deposition is a very useful tool. It can be used for generating a wide range of semiconductors both in the pure form and in the doped form. In order to make a metal oxide or a metal nitride, one can also use what is called plasma enhanced chemical vapor deposition, where there is a plasma which is which contains a reactive gas and in this plasma the chemical decomposes and this leads to the appropriate deposition of the appropriate material. This is also being found and it has been utilized very advantageously. In most of these cases that we have talked about today, organometallic compounds are used as materials and in some instances in recent times it has been used as a catalyst for making a material. In other words the organometallic itself is not part of the material that is formed, but it is used in very small amounts. So, that the material is generated in a pure form and in a useful material is generated. There is another example where a carbene has been used as a catalyst. This is an organic compound. This organic compound has been used as a catalyst for an organometallic reaction. So, this is the inverse of what we have been seeing so far. We have been using organometallic catalysts from carrying out organic reactions. Now, we are going to talk about two examples where organometallic compounds are used as catalysts in very unique ways. The first instance that I want to talk about is a single wall carbon nanotube. Single wall carbon nanotube is often formed by something very close to the chemical vapor deposition that I talked about. It is conducted by pyrolysis of carbon rich compounds. When you pyrolyse the carbon rich compound you lose hydrogen and generate a carbon network which is like a tube. So, this is a carbon nanotube and multiple tubes are formed. What is interesting is that the best catalyst that is available for making this carbon nanotube especially single walled carbon nanotube is ferrosine which is this organometallic molecule. Let us see why this is the case. If you look at a piece of a carbon nanotube very often it is indicated as a series of hexagons which are connected with each other. Here is a long picture of a long carbon nanotube, single walled carbon nanotube that is shown for you here. Here is another picture which is dynamic but you can see that the carbon nanotube is primarily made up of hexagons which are connected with each other. Now, there are different types of carbon nanotubes and these nanotubes have got different conducting properties different properties which have been utilized for different applications. But in most of the nanotubes you will notice that it is the 6 membered ring which is being linked to one another. So, how are these carbon nanotubes attached to or how are these hexagons attached to each other and how are they formed in the first place. Although we indicate carbon nanotubes the middle part of the nanotubes what we do not realize is that the end of the nanotube there is in fact a cup that is attached. It is this cup that is responsible for the formation of this nanotube. In other words the cup is formed first and then the nanotube propagates along this direction growing along this direction. So, the cup is of great importance if you cannot make the cup then you cannot make the nanotube. So, let us take a closer look at this cup that we are talking about. It is often called a cap for the nanotube and here is how one thinks about making the carbon nanotube. If you have acetylene as the source for the carbon required for making the carbon nanotube you can think of capping or attaching three benzene rings around the cyclopentadiene ring. If you use only benzene rings it would not be possible to make the curved cup formed at the end of the nanotube. So, if you have a cyclopentadiene ring however you can attach three C 6 units around it to form this curved molecule. Although we have drawn it in a flat two dimensional form you can see how this molecule will be curved. We think of the fact that it is not possible to keep all five carbon atoms in a plane when you have only a single double bond inside the ring. So, let us take a look at what would happen. If you attach some more cyclopentadiene rings to this final structure and you will now understand why one needs ferrocene because ferrocene supplies this five membered ring which is not accessible when you use only acetylene as a source for the carbon. So, ferrocene turns out to be an excellent catalyst because to form this initial curved structure that we have indicated here it is important to have a nucleating center and that nucleating center comes from this cyclopentadiene ring of the ferrocene. Once it is formed of course it keeps growing and that is what is shown here schematically. First the formation of the cup is indicated and that happens by combining two or three units of this cyclopentadiene containing ring structures which are surrounded by C 6 H 6 units and then the hydrogens are lost and then again you end up with a curved structure which can now propagate in a linear fashion. So the cap is all essential for forming the nanotube and so that is why carbon nanotubes are mostly formed using ferrocene as a catalyst. Now, I will come to one other example where an organic catalyst has been discovered for an organometallic reaction. Although this is a novelty at the moment the principle behind this whole discovery is very important and that is if you have a molecule which can both give and take electrons give and protons then that molecule will be a good catalyst. Now, organometallic molecules were good catalyst because they could both accept electrons into vacant orbitals and give electrons into the anti-bonding orbitals of organic molecules. By doing so they facilitated many electron, electronic structure changes on the molecule or in other words reactions. Now, if you look at a carbene, a carbene has got if it is a carbene such as what is shown here then it has got a pair of electrons which it can donate but at the same time perpendicular to the pair of electrons you have a vacant orbital which can accept electron density. So, as a result carbene can function very similar to organometallic molecules and they can be catalyst and in this example it so happens that they are catalyzing an organometallic reaction. The organometallic reaction that has happened is a conversion of biscyclo octatetraenyl ion which is basically ion coordinated to two cyclo octatetraen units. In one case it will be three double bonds coordinated to it in the other case the ring is different only two other double bonds would be coordinated to it. Now, we have indicated an A here to show that there are two different ion atoms that are going to react. So, initially you have FeA and then a second molecule of the cyclo octatetraen complex comes in and that is labeled as FeB. So, there are two sources for the ion in the dinuclear complex and a dinuclear intermediate is formed. This molecule in which there is a carbene attached to an ion and two cyclo octatetraen units attached to the second ion atom is an intermediate it has also been isolated and characterized. The beauty of the study was that in most cases it was possible for them to show the NMR of the compound and also characterize some of the intermediates using X-ray crystallography. And the compound that they finally isolated as the most stable or the majority of the species was a tri ion tricyclo octatetraenyl compound. And this molecule is got a beautiful structure where it adopts 6 pointed star kind of a structure and you have a first organometallic cluster which does not have carbon monoxide. So, here is a molecule shown for you it is a simple reaction where a bis cyclo octatetraenyl ion molecule is transferred as converted into a tri ion tricyclo octatetraenyl compound. And this compound has got an unusual oxidation state you can see that the ion is in fact attached each ion is attached to a 5 carbon unit and a 3 carbon unit. You started out with a neutral species, but by oxidative coupling you have in fact oxidized the ion and coupled 3 cyclo octatetraenyl units in such a way that ion now has a formal oxidation state of plus 2. So, let us take a look at this 6 pointed star. Here is a 6 pointed star that I was telling you about it is a very beautiful structure that we will look at in a 3 dimensional fashion to appreciate the beauty of this molecule. Here is a molecule in 3 dimensions and you can see that each ion atom is coordinated to either 5 carbons on one side. Here are the 5 carbons that are interacting with the ion and on the lower side you will see that the same ion atom is interacting with only 3 carbons. So, on one side it has got 3 carbons and on the other side it has got 5 carbons. And if you look at it and through this axis you will notice that it forms a 6 pointed star structure. So, this beautiful molecule is in fact formed by interacting an organic molecule with an organometallic species. And it is the organic catalyst which generates this molecule very efficiently. In fact this whole reaction can be carried out in 95 percent yield with a turnover number of 9.5 with this n heterocyclic carbene. What is interesting is that this all the intermediates several of the intermediates have been characterized using variations in the structure of the alkyl groups which are attached to the nitrogen. So, here is a proposed catalytic cycle which just involves loss of cyclo octatetraene in a sequential fashion formation of an ion ion bond and the formation of the trinuclear intermediate. So, here are two intermediates which have been isolated and these are available in the Cambridge structure database. And in one case you have an ion atom which is coordinated to a carbons. And two cyclo octatetraenes and that is this molecule and in other case you have a molecule which is coordinated to two of these carbene units at the same time. So, that is this molecule which is shown here. So, with this let me conclude by saying that materials for making new organometallic materials can be used in a variety of ways very very organometallic compounds can be used as materials and also for generating new materials. The advantage of organometallic compounds is that they are accessible easily accessible easily made and they can be tailor made to function in a particular way. And this is because they can be easily functionalized any of these organic ligands can be converted to new ligands and new molecules can be generated. And to top it all they are extremely affordable which is also important when you are thinking of making new materials. Finally, let me conclude by talking about organometallics in general. We have talked about a variety of systems where organometallics have been used for organic synthesis. They are useful for making new molecules, new drugs, new molecules for materials and in the biological science also for both for diagnosis and for therapy. But in all these cases the advantages of the organometallic compounds make them fairly unique. You have the redox properties of the metal atom which you can play around with. You also have the organic structure which you can change very readily. Where are we going? Well it is now possible to have systems where you can have domino reactions which means you combine two different organometallic catalysts in such a way that you carry out a reaction. In a single pot you carry out two different reactions sequentially. So, these are called domino reactions and these domino reactions are possible because of organometallic compounds which are behaving as catalysts. This also leads to new reactions which have not been known earlier. In this lecture, in this series of lectures we have only considered some of the very basic and simple lectures, simple reactions that are possible with organometallic compounds. Finally, of course, asymmetric catalysis is an extremely important project that is ahead of the organometallic chemist. In order to make chiral molecules very efficiently, it is important to use chiral ligands and asymmetric catalysis will play a very important role in organometallic chemistry in the future also.